Investigation of the events occurring during the formation of neural connections has, heretofore, been pursued primarily on "model systems" such as in vitro cultures and regenerating systems. Whether these model systems reflect the events occurring during embryogenesis is uncertain, so the problem will be studied in the embryonic lobster (Homarus americanus). This system has a number of distinct advantages: 1) large numbers (over 10,000) of embryos are available at the same stage of development on a single animal; 2) the rate of development is easily controlled and indeed, can be stopped or progressed at will; 3) the neuromuscular system is in some ways similar to CNS (e.g., muscle fibers receive multiple inputs from excitatory and inhibitory neurons) but the anatomy is simple enough to lend itself to experimental analysis. Intracellular recording from embryonic abdominal muscles has demonstrated that neuromuscular synapses are active as early as the third month of a nine month development. Parallel ultrastructural studies have lead to the same conclusion. These results will be extended using electrophysiological, pharmacological and ultrastructural techniques. In particular, we will attempt to determine: 1) the pattern of innervation of the abdominal muscles in adult and post-larval animals; 2) the time course of growth of the abdominal ganglia, and the roots (nerve tracts) which serve the abdominal musculature; 3) the time course of innervation for the various axons to each of the abdominal musculature; 4) the events involved in formation and maturation of nerve-muscle connections; 5) the properties of the early muscle cell, including the similarities and differences between fast and slow muscle anlagen in regard to resting membrane potential, active membrane properties, ultrastructure, presence and location of receptors for the two transmitter substances, and finally, the differences in properties of the synapses on the two types of muscle fibers.